1. Technical Field
The present disclosure relates to a bulk acoustic wave (BAW) resonator.
2. Description of the Related Art
A BAW resonator comprises a resonator core, or piezoelectric resonator, formed of two electrodes between which is arranged a layer of a piezoelectric material. When an electric field is applied to the piezoelectric layer by application of a potential difference between the electrodes, this creates a mechanical disturbance in the form of acoustic waves. Such waves propagate in the resonator. The fundamental resonance settles when the acoustic wavelength in the piezoelectric material substantially corresponds to twice the thickness of the piezoelectric layer.
A resonator especially comprises two notable frequencies:
the frequency at which the acoustic impedance of the resonator is the smallest, currently called the resonance frequency; and
the frequency at which the acoustic impedance of the resonator is the largest, currently called antiresonance frequency.
BAW resonators are generally formed above a substrate, for example, on a silicon wafer. An acoustic isolation device is then provided between the resonator core and the substrate to avoid losing the acoustic waves in the substrate. There mainly exist two types of BAW resonators: suspended BAW resonators and BAW resonators mounted on a substrate.
Suspended BAW resonators, better known as FBARs (Film Bulk Acoustic Wave Resonators) or TFRs (Thin Film Resonators), comprise an isolating air layer between the resonator core and the substrate. There thus is a cavity in the substrate or a bridge above the substrate.
BAW resonators mounted on the substrate, better known as SMRs (Solidly Mounted Resonators), are isolated from the substrate by an acoustic reflector, currently called Bragg mirror. They have a stronger structure, better adapted to conventional manufacturing methods.
BAW resonators with a Bragg mirror are considered herein.
FIG. 1 is a cross-section view schematically showing a BAW resonator 1 with a Bragg mirror formed on a substrate 3. Although FIG. 1 shows a single resonator, in practice, many resonators are formed simultaneously on a same substrate wafer.
Resonator 1 comprises a piezoelectric resonator 5 formed of the stacking of a lower electrode 5a, of a layer 5b of a piezoelectric material, and of an upper electrode 5c. 
An isolation structure 7, for example, a Bragg mirror, forms an interface between piezoelectric resonator 5 and substrate 3. A reflector 7 is an alternated stack of layers 7a of a material with a low acoustic impedance and of layers 7b of a material with a high acoustic impedance, each of these materials being indifferently isolating or conductive.
The thickness of each layer 7a, 7b is selected to be substantially equal to one quarter of the resonance acoustic wavelength in the material forming it. At the resonance frequency, the reflector 7 behaves as an acoustic mirror and sends the waves back to the resonator. The quality of the acoustic isolation increases along with the number of layers 7a, 7b of the alternated stack. In practice, reflectors with 4 or 5 layers are frequently used.
FIG. 2 is a top view schematically showing BAW resonator 1 of FIG. 1. Contact pads 13 and 14 are connected by conductive tracks 15 and 16, to lower and upper electrodes 5a and 5c of piezoelectric resonator 5. Conductive tracks 15 and 16 are schematically shown in FIG. 1 as an extension of electrodes 5a and 5c. Region 11, between pads 13 and 14 and tracks 15 and 16, and over substrate 3, is generally filled with silicon oxide.
The surface area taken up by contact pads 13 and 14 and by interconnect tracks 15 and 16 is, in practice, generally close to the active surface area of piezoelectric resonator 5.
A disadvantage of the structure described hereabove is that it has a low quality factor as compared with the theoretical quality factor of a BAW resonator with a Bragg mirror.
It is here desired to improve the quality factor, around the antiresonance frequency, of a BAW resonator intended to be used in an oscillator.